when to start missed approach

[crosswindaviator]

hi,

could someone help me on this,

When are you obliged to start a missed approach?
say for example your on the glideslope of an ils, can you wait untill the minima? Or do you have to start the missed approach before the minima to be sure not going one foot under it??

greetz

[moggiee]

On a precision approach (ILS) you have a Decision Altitude (DA). On a non-precision approach (Localiser, VOR, NDB, SRA) you have a Minimum Descent Altitude (MDA). Basically the difference is as follows:

DA: You make a decision NO LATER THAN the DA and if that decision is "go-around" you commence an immediate go-around and Missed Approach Procedure (MAP). It is accepted that you will descend slightly below the DA even if you start the go-around perfectly on time, but that this dip is typically going to be less than 50'and will not adversely affect obstacle clearance. The dip below DA is factored into the performance planning calculations used during the creation of the approach.

MDA: You must NOT descend below this altitude at any time unless you have the appropriate minimum visual references required to continue visually. In order to avoid going below MDA you must effectively make the decision to discontinue the approach a little way above MDA in order to give yourself time to level off at or just above MDA (up to 100' above on your IRT!). You normally need to allow 50-100' above MDA to give yourself time to level off without breaking the MDA. If you decide to discontinue the approach on a non-precision approach, you can level at (or just above) the MDA and continue to track inbound in the hope that you will acquire the runway visually and therefore make a visual approach and landing. However, you may not continue inbound any further than the Missed Approach Point (MAPt) which will be specified on the approach plate (typically the threshold/0 DME) at which point you must commence a go-around.

In practice, many airlines (especially jet operators) do not level off and track to the MAPt - if they are not visual by the time they get to the MDA they commence an immediate go-around. This is because once you level a 737-type aeroplane (or larger!) at MDA the inertia or the aeroplane makes it highly unlikely that you will be able to effect a safe landing.

Hope that answers your question!

[BEagle]

Interesting point about some airlines flying non-precision approaches as though they were precision approaches. If they do that, then they cannot use the published MDA as the go-around point as they will inevitably descend below MDA if they do.

If the non-precision approach specifies a notional glidslope technique, levelling at MDA will inevitably put you above the notional glideslope as you level off. But given no 'notional glideslope' restriction, an approach about 50-100 ft below the 'altitude for range' values allows you to be at MDA safely and legally on a 3 deg non-precision approach.

We used to teach RAF VC10 pilots to brief the technique they were going to use on a non-precision approach very carefully. Personally, I would note the height for range values as called but would plan to be slightly below them if a notional glideslope was non-mandatory. The actual assumed maximum descent rate in the design of a non-precision approach is, if I recall correctly, 400ft per mile which corresponds to 1000 ft/min at 150 kts. So a normal 750 ft/min descent rate would be entirely safe 'paralleling' the published 3 deg approach about 50-100 ft below it. Also had the inbound NDB radial set on the FMS and in the 'K model the HSI source switch to INS to give back-up CDI cross-track error information as we only had old-fasioned RMIs to fly the NDB approach on.

On reaching MDH we certainly used to fly level until the missed-approach point rather than just going around. This was great sport when flying a simulated 3-engine NDB (not NDB/DME) approach to RW 26 at Brize by night. If practised at any time before 2300 local, it gave you a good legal reason for going around at full power from 670 ft QFE right overhead the Stn Cdr's house.....:E

Just to let him know that his mighty Vickers Whisperjet crews were still at work at that time of night - even if he wasn't. Not permitted nowadays though:mad:

[crosswindaviator]

thanks alot guys,

the: is accepted that you will descend slightly below the DA even if you start the go-around perfectly on time...
was the main answer to my question.

thanks alot :ok:

[Tinstaafl]

...but not if the procedure uses an MDA. An important distinction as was described above.

[DB6]

BEagle, in Loganair we use a 'CDA' (Company Decision Altitude) during non precision approaches, which is simply MDA + 50'. Bugs are set with reference to that and the go-around is flown immediately if not visual at CDA. No level segment as mentioned since we couldn't land safely from much above the glideslope and that's just a Saab 340. I can't imagine bigger aircraft could manage it either although that is during passenger carrying ops. It might be fun to try though :E .

[moggiee]

BEagle, the procedure described by DB6 is basically that used by BA - they publish company decision altitudes for all the runways/approaches which are approved for that aeroplane which (for a B737) are typically 50-60' higher that the state published MDA. If a go-around is commenced at the CDA, then the aeroplane will not descend below MDA in the course of the go-around. It works well.

Other airlines I have dealt with add a straight 50' the MDA and treat it at as a DA.

The aim on the way down, by the way, is to accurately replicate the "ILS" glideslope of 3º so that you arrive at the CDA with 2 reds and 2 whites on the PAPIs.

In my time on 10 Sqn, the procedure we were taught by 241 OCU was to level at the MDA and track to the MAPt before the GA - a complete waste of time as you can not safely land the beast from a threshold crossing height of 500'!!

[BEagle]

Yes, the CDA technique resolve the MDA question admirably for those airlines with a '3 deg and go-around at minima' SOP.

When we first had the VC10K, we used to fly the NDB to circle. We'd fly level on reaching MDH, then a 600 ft QFE visual approach. Not that difficult given adequate practice - but later banned beacuse of noise and because we could actually practise it in the new simulator.

Few airlines will have the luxury of giving their pilots sufficient manual flying practice to be able fly a 600 ft a.g.l visual approach comfortably, I would imagine?

[reynoldsno1]

A word of caution if the missed approach procedure involves a turn, especially at the MAPt. Unless there is a statement to the effect that turns before the MAPt are allowed, always transition through the MAPt before turning, even if you are well above the DA/MDA.

[moggiee]

BEagle - I suspect that you would struggle to find an airport big enough to take a 737 that would permit a 600ft QFE circuit!

BTW, BA actively encourage their aircrew to hand fly the aeroplane to keep their handling skills up.

[FlyingForFun]

I suspect that you would struggle to find an airport big enough to take a 737 that would permit a 600ft QFE circuit!The airport where I work has several B737s fly in and out each day. And I regularly teach students bad-weather circuits at 600ft QFE after coordinating with ATC (which rarely involves much more than me asking for a bad-weather circuit and ATC approving).

Or are you talking about doing a 600ft QFE circuit in a B737? ;)

FFF
-------------

[timzsta]

If you went around at DA on your IRT and subsequently dipped just below the DA you would fail.

[moggiee]

FFF - thank you for that. I did mean 600ft QFE circuits in a B737 class aeroplane (or larger).

timzsta - indeed you would fail an IRT for that, but you may dip below DA when commencing the GA. However, you must never, at any point, dip below MDA - I did it on an IR renewal and it cost me a retake.

[PPRuNeUser0172]

timz

You would not fail an IRT for momentarily going below DA whilst initiating a Go Around. I think Moggie is wrong above. A DA is exactly that, a decision point, not as in a Non precision app, a MINIMUM descent altitude which you would definitely fail an IRT for busting.

A missed app from a precision app will have the fact that you will sink below DA factored into it with regards terrain clearance etc.

Simple really isnt it..............!

[reynoldsno1]

A precision approach is designed to accommodate height loss at the DA - if an IRE fails you for going below the DA, tell him he's talking b*ll*cks...

[Tinstaafl]

I don't think Timzta is referring to dipping below the DA when commencing the missed approach, but once in the climb if the a/c should go below the DA again. Fair enough, I think.

But to fail someone for dipping below the DA when they're commencing the missed approach exactly in accordance with procedure design is not right.

[moggiee]

Please don't misquote me chaps. I said that going below DA during the initiation of a go-around is allowed, and indeed expected.

However, as Timzsta and I did say, if you descend below DA after commencing the GA then you WILL be in trouble!